The present invention relates to an automatic document conveying apparatus, and more particularly relates to an improvement in the apparatus by which the document is automatically conveyed to a reading position in a recording apparatus or an image reading apparatus of an electrophotographic copier.
Generally, the automatic document conveying apparatus is mounted on a platen glass of an image exposing section of the copier. In a travelling type optical system copier to which the automatic document conveying apparatus is mounted, documents are sent one by one by the automatic document conveying apparatus from a document tray (a stack section) on which the documents are stacked, and conveyed on the platen glass by a conveyance belt. Then, the document is exposed by a lamp of an exposing section, and after that, the document is conveyed to a document discharging tray.
In a document exchanging operation on the platen glass by the automatic document conveying apparatus, conventionally, after a sheet discharging sensor detects that the document on the platen glass has been discharged, the next document is fed from the stack section, conveyed onto the platen glass, and stopped at a predetermined position. In the foregoing, a long document exchanging time is necessary, and therefore, a long copy processing time is necessary, so that the copying efficiency is lowered.
At the time of the document exchanging operation by which the document is automatically fed onto the platen glass and discharged from it in the automatic document conveying apparatus, the copying efficiency is 100 %, and a long copy processing time is not necessary when the document is exchanged during the returning time of an optical scanning unit. Due to the foregoing, it is necessary to increase the document conveyance speed at which the document is conveyed from a document multi-feeding prevention position in the stack section to a document stop position on the platen glass.
However, when the document feeding speed is increased, the following problems are caused: a large-sized motor is necessary; electric power consumption is increased; noise is generated; the conveyance belt is worn out; a document is damaged; and the document separation properties are lowered.
As a technique to realize the document conveying apparatus by which the above-described problems are solved and the document exchanging time can be shortened without an increase of document conveyance speed, the assignee of the present invention have proposed an automatic document conveying apparatus structured as follows. (Japanese Patent Application No. 385/1992)
That is, a document is placed on the platen glass; succeeding documents are caused to stand by at predetermined waiting positions respectively with a predetermined distance therebetween; and these documents are sequentially placed in a row downstream from a separation feeding means, by which the document is separated and fed from the document tray, to the reading section on the platen glass.
In the above-described automatic document conveying apparatus, positioning of the documents with respect to the document reading position is carried out as follows: a driving motor is turned off after a detection timing by a document passage detecting sensor; and a leading edge of the document comes into contact with a document stopper protruded onto the upper surface of the platen glass. However, in the aforementioned method, the following problems are caused: a stop position of the document fluctuates, so that it is difficult to carry out highly accurate positioning stably; and it is necessary to provide a mechanism by which the document stopper is protruded onto or withdrawn from the upper surface of the platen glass.
Accordingly, the following method has been investigated: a document movement amount is detected when the number of pulses generated by a rotary encoder provided to a driving roller of the conveyance belt is counted on the basis of a passing time of the document by a register sensor; and when the number of counted pulses is equal to a predetermined number, a driving motor and an electromagnetic brake are controlled.
However, as described above, when a plurality of documents are sequentially placed in a row in order to shorten the document exchange time, especially in the case where the size of the document is small, there occurs a case where a plurality of documents are placed between the register sensor and the document reading position.
In this case, even when pulse counting is started in order to detect the stop position of the first document according to the signal of the sensor which has detected the passage of the first document, the next document passes through the sensor section before the number of counted pulses is equal to the number corresponding to the stop position. Therefore, in the structure in which a plurality of documents are placed on the conveyance path so that the document exchanging time is shortened, there occurs a case where the conveyance control by a simple pulse counting operation can not be applied to the system, which is a problem.